The present invention relates to load sensing flow control systems, and in particular, to such flow control systems including load sensing controllers having fluid actuated mechanisms for imparting follow-up movement to the controller valve means. Furthermore, the present invention relates to load sensing flow control systems in which the load sensing circuit itself is utilized to overcome problems associated with overrunning loads and cavitation.
Although the present invention relates to many types of load sensing flow control systems, the problems associated with overruning loads and cavitation are especially noticeable in load sensing hydrostatic power steering systems. Therefore, the present invention will be described in connection with such a load sensing steering system, and more particularly, in connection with a load sensing controller of the general type illustrated and described in U.S. Pat. No. 4,096,883, assigned to the assignee of the present invention.
Cavitation in hydrostatic power steering systems is frequently caused by the system being subjected to an overrunning load. This is generally understood by those skilled in the art and will be described only briefly. The term "overrunning load" as used herein typically refers to a situation in which an external force is applied to the steered wheels, and thus, also to the steering cylinder. When this occurs, the rate of cylinder travel may exceed the steering wheel rate, such that fluid is discharged from the cylinder, back to the system reservoir, at a rate faster than fluid is supplied to the cylinder, through the controller. The typical result will be that in the delivery side of the circuit, the fluid pressure drops below atmospheric pressure, causing the formation of a gas pocket. In controllers of the type to which the present invention relates, the gas pocket may be transferred from wherever it first occurs to the fluid actuated follow-up mechanism (fluid meter).
In conventional controllers for hydrostatic steering systems there have been two primary approaches to the problem of cavitation, summarized briefly as follows:
1. The use of anti-cavitation check valves to permit fluid to enter the delivery side of the circuit (when it drops below atmospheric pressure) from a source such as the return line; PA1 2. Modification the controller valving to restrict the rate of fluid discharge from the cylinder, through the controller.
Each of these approaches has been somewhat satisfactory, and can also be used upon load sensing controllers. However, neither of these approaches can guarantee keeping the flow rate through the delivery side of the circuit the same as the discharge flow rate. In other words, each of the prior art approaches does help, but neither is a "positive" solution to the problem, i.e., neither one inherently provides a known relationship between the discharge flow rate and the flow rate through the delivery side of the circuit.